Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Analysis of Change in Balance by Component of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	$ 4,566,861	$ 4,055,552
Other comprehensive income (loss) before reclassifications, net of tax	144,227	302,959
Reclassifications, net of tax	1,064	569
Other comprehensive income (loss)	145,291	303,528
Stockholders' equity, ending balance	4,744,827	4,391,743
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	692,314	332,333
Other comprehensive income (loss) before reclassifications, net of tax	140,700	303,094
Reclassifications, net of tax	(957)	(1,090)
Other comprehensive income (loss)	139,743	302,004
Stockholders' equity, ending balance	832,057	634,337
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(6,682)	(5,115)
Other comprehensive income (loss) before reclassifications, net of tax	(500)	(1,800)
Reclassifications, net of tax	0	0
Other comprehensive income (loss)	(500)	(1,800)
Stockholders' equity, ending balance	(7,182)	(6,915)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	4,967	3,627
Other comprehensive income (loss) before reclassifications, net of tax	3,786	1,220
Reclassifications, net of tax	0	0
Other comprehensive income (loss)	3,786	1,220
Stockholders' equity, ending balance	8,753	4,847
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(113,025)	(98,898)
Other comprehensive income (loss) before reclassifications, net of tax	241	445
Reclassifications, net of tax	2,021	1,659
Other comprehensive income (loss)	2,262	2,104
Stockholders' equity, ending balance	(110,763)	(96,794)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	577,574	231,947
Stockholders' equity, ending balance	$ 722,865	$ 535,475